One of the most respected Eurosceptic Tory MPs today launched a campaign for a referendum on the Lisbon treaty in a direct challenge to the party leadership which last week said it would not make good David Cameron's pledge for a poll on the issue.

In an email to constituents announcing his intention to campaign for a referendum, Douglas Carswell said: "I want you to know that I have begun a campaign for a referendum on the EU. All three parties promised us a referendum. Yet somehow it hasn't happened. I think that's wrong."

Last week Cameron disappointed Tory MPs and grassroots alike when he ditched his commitment to hold the poll on the grounds that a pledge made regarding a treaty was no longer relevant once the treaty had become law. Instead he promised to prohibit any further transfer of power to the EU without a referendum, and to negotiate to bring certain powers back to Britain.

Today this new policy position was undermined when Kenneth Clarke, the Europhile shadow business secretary, described the new policy as "largely reassurance".

He said: "We're going to have a go at repatriating, going back to the old opt-out on the social chapter. People want to be reassured on criminal justice. I think it's largely reassurance, myself."

Chris Bryant, the minister for Europe, said: "The cat's out of the bag. David Cameron's leadership has been found wanting on Europe and Tory divisions are clearly alive and well. Ken Clarke's comments show he doesn't believe Cameron's rhetoric and it's certainly not enough for his own backbenchers, who are launching a campaign against their leader's policy."

In the immediate aftermath of his decision Cameron was hit by the resignation of two hardline Eurosceptic MEPs, Roger Helmer and Daniel Hannan. Carswell's launch of a campaign for a referendum indicates that not all Tory backbenchers will accept the policy change.

Carswell – whose last campaign to have the former Speaker Michael Martin removed from office ended in success – was bullish about his chances of securing a vote on a referendum, telling his constituents: "I will campaign for a vote on Europe with the same determination that I campaigned to remove the Commons Speaker.

"No one in Britain under the age of 52 has had the chance to vote in a referendum on Europe. For years it has been left to professional politicians and diplomats to decide EU policy. I believe it is now time to let the people have their say."

In an interview at the weekend Carswell said he continued to support Cameron, but had a different opinion to his party high command on the issue.